Vitamin D: a meal containing fat improves absorption

Taking vitamin D supplements during a meal containing fats, either monounsaturated or polyunsaturated, can increase absorption by 32% compared to when the same dose of the vitamin is taken without lipids.

This was found by the authors of a study conducted at Tufts University in Boston (USA) and published in the Journal of the Academy of Nutrition and Dietetics.

What is vitamin D

More than a true vitamin, vitamin D is a pro-hormone. It gets its nickname ("sunshine vitamin") from the fact that it can be synthesized by the action of sunlight on the skin. Its precursor (7-dehydrocholesterol) is first converted into pre-vitamin D and then into vitamin D3 (or cholecalciferol) – which is the form of vitamin D produced by the human body.

As synthesized in the skin, vitamin D3 is inactive. To perform its functions, it must be modified in the liver and kidneys. The liver produces 25-hydroxyvitamin D3 (25(OH)D), which is the form measured in blood tests to assess vitamin D3 levels. The kidneys (along with some other tissues) produce the active form of this pro-hormone, 1,25-dihydroxyvitamin D3 (1,25(OH)D), which exerts its actions by interacting with its receptors.

Vitamin D3: what is it for? Properties

Vitamin D3 has multiple properties:

• in the pancreas, it increases the secretion of insulin, the hormone that controls blood sugar levels (glycemia);
• in the bones, it participates in bone metabolism and mineralization;
• in the intestine, it increases the absorption of calcium and phosphorus;
• it regulates cell proliferation and differentiation;
• it participates in the regulation of the immune system.

According to the European Food Safety Authority (EFSA), a vitamin D3 supplement can claim these benefits:

• vitamin D is necessary for the normal growth and development of bone in children;
• vitamin D helps reduce the risk of falls associated with postural instability and muscle weakness. Falls are a risk factor for bone fractures in men and women aged 60 and over;
• vitamin D contributes to normal calcium and phosphorus absorption/utilization;
• vitamin D contributes to maintaining normal blood calcium levels;
• vitamin D contributes to maintaining normal bones;
• vitamin D contributes to the maintenance of normal muscle function;
• vitamin D contributes to maintaining normal teeth;
• vitamin D contributes to the normal functioning of the immune system;
• vitamin D contributes to the normal functioning of the immune system in children;
• vitamin D plays a role in the process of cell division.

Blood levels of vitamin D3 are considered normal if at least 30 ng/ml. When 25(OH)D is between 20 and 29 ng/ml, it is considered insufficient, while below 20 ng/ml it is a deficiency state (very severe if the measured value is below 7 ng/ml). When vitamin D3 is low, symptoms such as bone pain, muscle pain, and muscle weakness may appear.

Given the link between a nutrient known for its bone benefits, calcium, and vitamin D3, it is not surprising that in children vitamin D3 deficiency causes rickets and in adults leads to osteomalacia, a condition where the collagen matrix inside the bones is inadequately mineralized, making the bone weak and unable to provide the structural support it needs, thus increasing fracture risk. Moreover, vitamin D deficiency prevents children from optimizing bone development and hinders their growth.

Sources of vitamin D3: where to find it

It is clear how important it is to meet the body's daily vitamin D needs. Unfortunately, nowadays this task seems far from simple.

Foods containing vitamin D3 are few; mainly certain types of fish (such as herring, mackerel, and salmon), cod liver oil, butter, some cheeses, and eggs. Some plant-based foods, like mushrooms and chocolate, also contain vitamin D, but in a different form: ergocalciferol, also called "vitamin D2."

Both forms of vitamin D are converted by the liver and kidneys into their active forms, but there is still some doubt whether active vitamin D2 has the same properties as active vitamin D3. And the differences don't end there, because vitamin D2 has a harder time binding to the main protein that allows vitamin D (a fat-soluble substance) to be transported in plasma (an aqueous solution). Also, vitamin D2 is degraded more rapidly by the body.

Considering all these aspects, dietary vitamin D is generally not sufficient to meet the body's needs. While in Mediterranean countries the ultraviolet radiation from sunlight is usually enough to cover vitamin D needs, in some northern countries sun exposure is very weak. Moreover, even in Mediterranean countries people often spend too much time indoors, significantly reducing sun exposure.

To address this situation, it is useful to take vitamin D3 supplements or fortified foods (i.e., foods enriched with vitamin D3). Some studies suggest that taking vitamin D3 is a more effective strategy than using vitamin D2 supplements. Additional experiments indicate that vitamin D2 intake might reduce blood levels of vitamin D3; the consequences of this effect are still unknown.

Today, vitamin D supplements are, after multivitamins, the most widely used dietary supplements. Prescribing vitamin D3 to newborns is routine, and supplementation is recommended throughout the first year of life. Many adults also take it, precisely because many lead lifestyles that strongly reduce direct skin exposure to sunlight, favoring vitamin D3 deficiency.

Since vitamin D is a fat-soluble nutrient, it was hypothesized that foods containing fats could influence its absorption. Before the study published in the Journal of Academy of Nutrition and Dietetics, however, there was little clear information on this.

The right amount of fat improves vitamin D absorption

In the study, Bess Dawson-Hughes and colleagues recruited 50 healthy elderly men and women. Participants were divided into three groups: the first group consumed meals without fat, while the second and third consumed meals in which 30% of calories came from fat, but differing in one aspect: the ratio of monounsaturated fatty acids (MUFA) to polyunsaturated fatty acids (PUFA). The second group’s meals had a low MUFA:PUFA ratio (1:4), while the third group’s meals had a high ratio (4:1).

After a 12-hour fast, all participants had blood drawn, ate breakfast, and took a vitamin D3 supplement (50,000 IU, about 125 micrograms). After 10 hours (during which they ate nothing else), all had blood drawn again and then ate dinner; after 2 and 4 more hours, further blood samples were taken from all participants.

Blood analysis showed that 12 hours after taking the vitamin D3 supplement, plasma vitamin D levels were 32% higher in subjects who had consumed foods containing fat. Absorption was also higher at 10 and 14 hours after breakfast (40% and 25%, respectively), but did not appear to be influenced by the MUFA to PUFA ratio.

A very useful discovery

These results helped to better understand how meal composition can influence the absorption of a vitamin D3 supplement and suggest it is better to take vitamin D with meals containing fats.

How much vitamin D3 to take daily depends on individual needs: a doctor or nutritionist will define it case by case. Moreover, the most suitable formulation can depend on specific requirements: sometimes a vitamin D3-only supplement is enough, other times a combination of calcium carbonate plus vitamin D3 is better, and sometimes it is more useful to take vitamin D3 and Omega 3 from krill oil, an ideal pairing to simultaneously protect bones, the immune system, and the cardiovascular system.

In any case, it is advisable not to exceed the maximum tolerable intake level indicated by the Italian Society of Human Nutrition (40 micrograms for infants, 65 micrograms from 1 to 3 years, 75 micrograms from 4 to 10 years, and 100 micrograms from 11 years onward). Excess vitamin D3 can cause side effects such as nausea, vomiting, constipation, loss of appetite, weight loss, weakness, confusion, disorientation, kidney damage, kidney stones, and calcium deposits in soft tissues (for example, in the heart and lungs).

References:

Chauhan K et al. Vitamin D. [Updated 2023 Apr 9]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2024 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK441912/

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Dawson-Hughes B et al. Dietary fat increases vitamin D-3 absorption. J Acad Nutr Diet. 2015 Feb;115(2):225-230. doi: 10.1016/j.jand.2014.09.014

Md Isa Z et al. The Impact of Vitamin D Deficiency and Insufficiency on the Outcome of Type 2 Diabetes Mellitus Patients: A Systematic Review. Nutrients. 2023 May; 15(10): 2310. doi: 10.3390/nu15102310

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